This invention relates to a device for feeding printing plates onto the plate cylinder of a printing machine.
In certain types of sheet-fed offset printing machines, a printing plate is fastened on the plate cylinder by means of two tension rails located in a channel in the cylinder and assigned respectively to the print-start region and to the print-end region of the plate. Initially, the print-start region of the printing plate is inserted into and clamped within the gripping region of the corresponding tension rail. Thereafter, the print-end region of the plate is likewise clamped in the corresponding second tension rail, the plate then being tensioned by the exertion of force. When large-format printing plates are used, the introduction of the print-start region of the printing plate into the corresponding print-start tension rail is very laborious and time-consuming since the corresponding edge of the printing plate has to be introduced over its entire width into a sometimes very narrow slot of the gripping region. A waviness of the print-start edge of the printing plate makes this situation even worse. Also, during the operation of drawing on the printing plate, that is to say during the slow forward run of the plate cylinder, and during the drawing off of an old printing plate upon backward running of the cylinder, the printing plate has to be held or supported so as not to rest with its printing side on any machine parts or else the plate may be damaged.
German Patent Specifications Nos. DE 3,940,795-C2 and DE 3,940,796-C2 and European Patent Specification No. EP 0,431,575-A2 disclose systems for an automatic printing plate change in which the drawing off of an old printing plate from and the feeding of a new printing plate to the plate cylinder take place automatically. In addition to storage and reception regions for the printing plates, these systems also have transport devices for feeding the printing plates and conveying them away. These systems, however, are of complex construction and are therefore relatively expensive.
A semi-automatic printing plate changing system in which a new printing plate is fed to the plate cylinder from a position located on an upwardly pivotable guard in front of the printing-unit cylinder is also known from Japanese Patent Specification No. JP-A-59-22755. Page 6 of the February, 1992 issue of "Der Polygraph" ["The Polygraph"] and Japanese Patent Specification No. JP-A-63-191636 disclose semi-automatic printing plate changing systems in which the plate cylinder has devices for clamping the print-start region and the print-end region of the printing plate and for then tensioning the plate. Also provided are devices by which the printing plate can be drawn onto the plate cylinder with simpler handling techniques. The device disclosed in the last-mentioned Japanese Patent Specification provides, in particular, a pivotably mounted press-down roller which can be advanced up to the outer circumference of the plate cylinder and with which the print-end region of the printing plate is introduced into the corresponding print-end tension rail. However, a device for substantially simplifying the operation of introducing the front edge of the printing plate into the corresponding tension rail is not known.